Larry Sanger

He is currently organizing a rival, expert-edited wiki-based encyclopedia, known as Citizendium.

Sanger's role in the founding of Wikipedia is a matter of dispute. Although Sanger came up with the idea of using wiki software to host their encyclopedia, Wales claims to be the sole "founder" of Wikipedia.
Citizendium's FAQ page, presumably speaking for Sanger, describes him as a "Wikipedia co-founder." Digital Universe says, "He was the chief organizer of Wikipedia in its first year and played a key role in its creation." [1]

Wired says:

Wales started an Internet company called Bomis, a search engine and Web directory. He began hearing about the fledgling open source movement and wondered whether volunteers could create something besides software. So he recruited Larry Sanger, then an Ohio State University doctoral student in philosophy, whom he'd encountered on some listservs. He put Sanger on the Bomis payroll, and together they launched a free online encyclopedia called Nupedia. [2]